Paradox
It seems that time travel may be a real possibility, given that there exits exotic matter and a wormhole can be constructed. But there is another rather compelling reason that many people do not believe that time travel is possible. If time travel were possible paradoxes would be allowed to exist and that most cherished notion of causality could be violated. Some have speculated that the universe may have a law that prevents causality violation or paradox. Some, however, believe that maybe we cannot travel to our own past and cause paradox but we can travel to the past of a parallel universe.